The present invention pertains in a general manner to the field of image processing, and more precisely to the coding and to the decoding of digital images and of sequences of digital images.
Digital images and sequences of digital images are known to occupy a great deal of memory space, thus making it necessary, when these images are transmitted, to compress them so as to avoid problems of crowding on the communication network used for this transmission, the throughput usable thereon generally being limited.
The H.264/MPEG-4 AVC standard (AVC standing for “Advanced Video Coding”), such as presented in the document ISO/IEC 14496-10, describes in particular a technique according to which groups of pixels, called macroblocks, of a current image are predicted spatially with respect to other macroblocks belonging to the same image. This is what is called Intra coding (or “Intra-frame-coding”). After this predictive coding, the blocks of pixels are transformed, by a transformation of discrete cosine transform type, and then quantized. The coefficients of the quantized blocks of pixels are thereafter traversed in an order of reading making it possible to utilize the significant number of zero coefficients in the high frequencies, and are then coded by an entropy coding.
More precisely, during the predictive coding of a macroblock in accordance with the aforementioned standard, with the exception of a macroblock of 16×16 type, the macroblock is partitioned into blocks of smaller size, whose form is rectangular or square. The spatial prediction of such a macroblock in an image consists in predicting each block of smaller size forming this macroblock with respect to one or more blocks of another macroblock of this same image. This prediction is possible only if this other macroblock is a macroblock which neighbors the macroblock to be predicted and lies in certain directions which are predetermined with respect to it, that is to say generally above and to the left, in a so-called “causal” neighborhood.
Thus for example, in the case of a macroblock of 16×16 type, the prediction of the pixels which are situated at the bottom, to the right of this macroblock, is necessarily performed with respect to reference pixels which are situated above and to the left of the macroblock. Such reference pixels are very distant spatially from the pixels to be predicted.
The accuracy of such a type of prediction is therefore not optimized, especially in the case of macroblocks having high spatial activity, that is to say in the case where the image to be coded exhibits numerous details.
The present invention is aimed at solving the drawbacks of the prior art by providing methods and devices for coding and decoding images, in which the prediction distance between the pixels to be predicted and the reference pixels is minimized.